Electrical system and method for protecting a DC/DC converter

ABSTRACT

The purpose of the present invention is an electrical system allowing conversion of a direct voltage into another direct voltage, including:
         a resonant DC-DC converter including an LLC converter circuit,   a control unit including:
           a first module for determining the rms resonance current value from a measurement of the output current,   a second module for determining a maximum value of the voltage at the terminals of each resonance capacitor and a minimum value of the voltage at the terminals of each resonance capacitor using rms resonance current value,   a comparison module,   a disconnection element configured to stop operation of the resonant DC-DC converter in the event of an overload.

TECHNICAL FIELD

The present invention concerns the field of systems for poweringelectrical and/or electronic equipment, in particular such systemsintended to be installed in a motor vehicle, in particular an electricor hybrid motor vehicle. The present invention concerns morespecifically the field of DC-DC converters, i.e. electrical systemsenabling a direct-current input voltage to be converted into adirect-current output voltage which is lower or higher than the inputvoltage.

In a known manner, an electric or hybrid motor vehicle includes anelectric motor system, powered by a high-voltage power battery via ahigh-voltage on-board electrical network, and various auxiliaryelectrical devices, powered by a low-voltage power battery, via alow-voltage on-board electrical network.

BACKGROUND

FIG. 1 represents a functional block diagram of an on-board electricalsystem of the state of the art. The HV high-voltage power battery thusimplements the function of supplying the electric motor system ENG withenergy, enabling the vehicle to be propelled. The LV low-voltage powerbattery powers the auxiliary electrical devices AUX, such as on-boardcomputers, electric window motors, the multimedia system, etc. The HVhigh-voltage power battery typically delivers a voltage of between 100 Vand 900 V, and preferably between 100 V and 500 V, while the LVlow-voltage power battery typically delivers a voltage of the order of12 V, 24 V or 48 V. These two high- and low-voltage power batteries, HVand LV, must be able to be charged.

Recharging the HV high-voltage power battery with electrical energy isaccomplished in a known manner by connecting it, via an electricalcharger of the vehicle, to an external electrical power network, forexample the G1 domestic AC electrical network. Finally, again withreference to FIG. 1, charging of the LV low-voltage power battery isaccomplished in a known manner by the HV high-voltage power battery. Forthis purpose the system includes a DC10 DC-DC converter.

The electrical charger typically includes an insulated DC-DC converter.A resonant LLC converter is known, illustrated in FIG. 2, comprising tworesonance capacitors C3, C4, a resonance coil L1 and a transformer. Ifthe output power of the circuit increases the resonance current of thetransformer also increases and, finally, the amplitude of voltage Vr atthe terminals of each resonance capacitor C3, C4 increases. When voltageamplitude variations Vr are too great they can cause overloads inresonance capacitors C3 and C4 and at the output of the circuit.

In a known manner, with reference to FIG. 3, to prevent potential damageto the circuit due to an overload, resonance capacitors C3 and C4 mustbe protected by limiting the voltages at their respective terminals. Toaccomplish this a first solution consists in placing “ultrafast” diodes,a reference to diodes which switch at very high frequencies, as in thepresent case, at over 275 kHz, in parallel with resonance capacitors C3and C4. Thus, when voltage Vr is positive the diode is off, but whenvoltage Vr is negative the diode is on, and short-circuits thecapacitor. This modifies the structure of the circuit, which istherefore no longer an LLC circuit, and no longer operates as one, thuspreventing resonance capacitors C3 and C4 from being overloaded.

This solution has disadvantages: in particular the high cost of these“ultrafast” diodes, bearing in mind that two such diodes are requiredfor each circuit. In addition, the two “ultrafast” diodes cannotshort-circuit at the same time. Finally, the diode's short-circuitthreshold voltage is not adjustable since it is intrinsic to the diode.

To mitigate these disadvantages the present invention proposes anelectrical system configured to use a method to detect the overloadingof the resonance capacitor, based on a current measurement.

SUMMARY

More specifically, the invention refers to an electrical system enablinga DC voltage to be converted into another DC voltage, including:

-   -   a resonant DC-DC converter including a resonant LLC converter        with a resonance inductor, two resonance capacitors and a        transformer,    -   a control unit including:        -   a first module for determining the average value of the            output current of the resonant DC-DC converter over a period            called the “evaluation period”,        -   a second module for determining a maximum value of the            voltage at the terminals of each resonance capacitor, and a            minimum value of the voltage at the terminals of each            resonance capacitor over the evaluation period, using the            average value of the output current,        -   a module to compare a maximum voltage threshold with the            maximum value of the voltage at the terminals of each            resonance capacitor and to compare a minimum voltage            threshold with the minimum value of the voltage at the            terminals of each resonance capacitor,        -   a fault element configured to detect a failure of the            resonant DC-DC converter if:            -   the said maximum voltage value (Vr_max) is greater than                or equal to the maximum voltage threshold                (Vr_define_max), or            -   the said minimum voltage value n (Vr_min) is less than                or equal to the minimum voltage threshold                (Vr_define_min).                This system enables an overload to be detected simply                and effectively.

Advantageously, the electrical system's resonant DC-DC converterincludes a rectifier, connected to the output of the transformer, and inparticular to the transformer's secondary winding.

The said rectifier enables the square wave AC voltage, at the output ofthe transformer, to be converted into a pulsed rectified voltage, i.e. avariable voltage, but with a constant sign.

The evaluation period is preferably equal to one or more switchingperiods of the transistors of the resonant DC-DC converter.

In a preferred manner the first determination module is configured todetermine the average value of the output current, from a measuringpoint located at an output terminal of the rectifier, in particular alow-output terminal of the rectifier.

The second module for determining the electrical system is preferablyconfigured to determine the maximum voltage value and the minimumvoltage value from the input voltage of the resonant DC-DC converter,the rms value of the resonance current, the switching frequency of theresonant DC-DC converter, and the value of the resonance capacitor.

The maximum voltage value and minimum voltage value are thus determinedprecisely.

Advantageously, the electrical system includes an electrical filterconnected to the output of the rectifier of the resonant DC-DCconverter.

The filter enables the quality of the signal being output from the DC-DCconverter to be improved.

Advantageously, the electrical system's comparison module is configuredsuch that:

-   the said maximum voltage threshold is a maximum authorised voltage    value at the terminals of the resonance capacitor, above which in    particular the said capacitor is overloaded, and-   the said minimum voltage threshold is a minimum authorised voltage    value at the terminals of the resonance capacity, below which in    particular the said resonance capacitor is overloaded.

The maximum voltage threshold and the minimum voltage threshold can bemodified, and thus be modified to suit the context of the system.

Advantageously, the electrical system's fault element includes adisconnection element configured to stop operation of the resonant DC-DCconverter in the event of a fault.

The invention also concerns an method for detecting an overload of aresonant DC-DC converter used in an electrical system including aresonant DC-DC converter including a resonant LLC converter circuitwhich includes a resonance inductor, two resonance capacitors and atransformer, where the said method is characterised by the fact that itincludes steps of:

-   -   determination, in particular of measurement, of the output        current,    -   determination of the average value of the said output current,        over a period known as the “evaluation period”,    -   determination of a maximum value of the voltage at the terminals        of each resonance capacitor, and a minimum value of the voltage        at the terminals of each resonance capacitor over the evaluation        period, using the average value of the output current,        determined in the previous step,    -   comparison of a maximum voltage threshold with the maximum value        of the voltage at the terminals of each resonance capacitor and        comparison of a minimum voltage threshold with the minimum value        of the voltage at the terminals of each resonance capacitor,    -   detection of a fault in the resonant DC-DC voltage converter if        the said maximum voltage value is greater than or equal to the        maximum voltage threshold, and/or if the said minimum voltage        value is less than or equal to the minimum voltage threshold.

In a preferred manner, the method's fault-detection step is thedetection of an overload of a resonance capacitor.

Advantageously, the method includes, after the fault-detection step, astep of disconnection of the resonant DC-DC converter, in whichoperation of the resonant DC-DC converter is stopped.

The method enables the electrical system, and in particular the DC-DCconverter, to be protected, in order to prevent possible damage tocertain components of the system, in particular the resonancecapacitors, and therefore to prevent erroneous operation of the system.

In a preferred manner, the maximum value of the voltage at the terminalsof each resonance capacitor is determined using the following formula:

$V_{r\_\max} = {{\frac{1}{2}V_{in}} + {\frac{\sqrt{2}}{C_{r}2\;\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$and the minimum value (Vr_min) of the voltage at the terminals of eachresonance capacitor (Cr/2) is determined using the following formula:

$V_{r\_\min} = {{\frac{1}{2}V_{in}} - {\frac{\sqrt{2}}{C_{r}2\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$where Vin is the input voltage of the resonant DC-DC converter, Cr isthe value of the resonance capacitors, Fs is the switching frequency ofthe resonant DC-DC converter, N refers to the transformer'stransformation ratio, Is_avrg is the average value of the outputcurrent, Vout is the output voltage and Lm refers to the transformer'sprimary magnetizing inductance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription, given only as an example, which makes reference to theappended drawings, given as non-restrictive examples, in which identicalreferences are given to similar objects, and in which:

FIG. 1 (previously commented), the functional block diagram of anelectrical system according to the state of the art;

FIG. 2 (previously commented), an electronic diagram of an electricalsystem according to the state of the art;

FIG. 3 (previously commented), an electronic diagram of an electricalsystem according to the state of the art;

FIG. 4, the functional block diagram of an electrical system accordingto the invention,

FIG. 5, the electronic diagram of the electrical system of FIG. 4,

FIG. 6, a block diagram representing the overload detection methodaccording to the invention.

It should be noted that the figures explain the invention in detail inorder to implement the invention, and that the said figures can ofcourse be used to improve the definition of the invention, ifapplicable.

DETAILED DESCRIPTION

It should be noted that the present invention is described below usingdifferent non-restrictive implementations, and may be implemented usingvariants within the understanding of a person skilled in the art, towhich the present invention also refers.

FIG. 4 represents a functional block diagram of a form of implementationof the electrical system according to the invention. This electricalsystem is intended, in particular, to be installed in an electric orhybrid motor vehicle. The invention concerns a resonant DC-DC converter1.

With reference to FIG. 4, the electrical system includes a resonantDC-DC converter 1, including a converter circuit 10, a rectifier 20,connected to the output of said converter circuit 10, and a filter 30,connected to the output of said rectifier 20. Filter 30 can be omitted.Resonant DC-DC converter 1 enables a DC voltage to be converted intoanother DC voltage; the detailed topology of this converter will bedescribed in detail in a later part. Converter circuit 10 includes afirst circuit 10-1 and a second circuit 10-2, enabling, in particular, asquare wave AC voltage to be obtained, or in other words a sinusoidalcurrent, from a DC voltage. Rectifier 20 enables a square wave ACvoltage to be converted into a pulsed rectified voltage, i.e. a variablevoltage, but with a constant sign. Filter 30 enables the voltageobtained in the previous step to be “smoothed”, i.e. enables the averagevalue of the input voltage of filter 30 to be obtained at the output offilter 30.

FIG. 5 represents the detailed topology of the electrical systempresented in FIG. 4, i.e. of a resonant DC-DC converter 1. In thisdetailed implementation first circuit 10-1 of converter circuit 10 ofresonant DC-DC converter 1 includes a circuit called an “HBS” circuit,where this acronym stands for “Half Bridge Switches”. In addition,second circuit 10-2 of converter circuit 10, connected to first circuit10-1, includes a resonant LLC converter circuit, the structure of whichis known to a person skilled in the art.

The HBS circuit includes two transistors T1 and T2, in particularfield-effect transistors, and performs as a switch mode power supply,with transistors T1, T2 operating in switching mode. Losses may occur onactivation and deactivation of each transistor T1, T2. Capacitors C1, C2can be connected respectively in parallel with transistors T1, T2 toenable zero-voltage switching (ZVS), and to minimize losses due toswitching, and thus to obtain a higher switching frequency fortransistors T1 and T2. Again with reference to FIG. 5, the resonant LLCconverter of second circuit 10-2 includes a resonance inductor Lr, tworesonance capacitors Cr/2, a first capacitor of which is connected withan high terminal of first circuit 10-1, and a second capacitor of whichis connected to a low terminal of first circuit 10-1, the two resonancecapacitors Cr/2 being connected to their other terminal in a middlepoint, and a transformer Tr, which has a magnetizing inductor in theprimary winding.

Rectifier 20 can be a four-diode bridge to allow voltage rectification.Indeed, a square wave AC voltage, changing from positive to negative, isrectified as a periodic voltage of constant sign, either positive ornegative.

In addition, again with reference to FIG. 5, filter 30 can include aresistor R1 and a capacitor C3 mounted in parallel, or simply acapacitor mounted in parallel with rectifier 20 or alternatively an LCfilter.

When the input voltage of filter 30, corresponding to the output voltageof rectifier 20, increases, capacitor C3 is charged. Then, when theinput voltage of filter 30 is reduced, capacitor C3 discharges. But, ina known manner, a capacitor is charged and is discharged “slowly”, andtherefore the amplitude of the voltage delivered at the output of filter30 is much lower than that of the input voltage of filter 30, or almostzero. The voltage at the output of filter 30 is thus almost adirect-current voltage.

Detection of a potential overload of one of resonance capacitors Cr/2 isaccomplished by measuring output current Is at a measurement point B1taken at the output of rectifier 20.

In addition, to detect a potential overload of a resonance capacitorCr/2, the electrical system includes a control unit TN. Control unit TNis in particular a digital processing device. Said control unit TNincludes a first determination module TN1, a second determination moduleTN2, including a comparison module TNC, and a fault detection elementUP. Control unit TN is connected, in particular, by its firstdetermination module TN1 to measurement point B1. Fault detectionelement UP can be a transistor control unit, commonly called a “driver”by a person skilled in the art.

With reference to FIG. 6, an implementation of the method for detectingan overload of a resonant DC-DC converter 1 is represented.

Step 1: Determination of the Rms Resonance Current Value

Ir_RMS

First determination module TN1 determines average value Is_avg of outputcurrent Is, measured at a measuring point B1 at a low output terminal ofrectifier 20 of resonant DC-DC converter 1, over a period T known as the“evaluation period”. This evaluation period T can extend over one ormore switching periods of transistors T1, T2 and can include a portionof a switching period.

Average value Is_avg of output current Is is also related to rmsresonance current value Ir_RMS.

Indeed, by applying Kirchhoff s current law to the node located at thelow terminal of the primary magnetizing inductor of transformer Tr, rmsvalue of the resonance current Ir_RMS is defined by the followingformula:I _(r_RMS)≈√{square root over (I _(s) ² +I _(m) ²)}  (1)It should be noted that Is refers to the measurement of the outputcurrent and Im refers to the current in the primary magnetizing inductorof transformer Tr.

Then, when the values of output current Is and of the current in theprimary magnetising inductor Im are replaced by their respectiveexpressions, in expression (1), the following expression is obtained:

$\begin{matrix}{I_{r\_{RMS}} \approx \sqrt{\left( {N \times I_{s\_{RMS}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}} & \left( {1{bis}} \right)\end{matrix}$It should be noted that N refers to the transformation ratio oftransformer Tr defined mathematically by the ratio of the number ofcoils of the secondary inductor to the number of coils of the primaryinductor of transformer Tr. In addition, Lm refers to the primarymagnetizing inductor of transformer Tr, Vout refers to the outputvoltage of rectifier 20, Fs refers to the switching frequency ofresonant DC-DC converter 1, and therefore in the present case theswitching frequency of transistors T1, T2 and IS_RMS defines the rmsvalue of the output current.

It is also known that rms output current value Is_rms is defined usingthe following formula:

$I_{s\_{RMS}} = {I_{s\_{avg}} \times {\frac{\pi}{2\sqrt{2}}.}}$Thus, if in (1 bis) rms output current value Is_rms is replaced by itsexpression, one obtains:

$\begin{matrix}{I_{r\_{RMS}} \approx \sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}} & \left( {1{ter}} \right)\end{matrix}$Thus, equation (1 ter) demonstrates that average value Is_avg of outputcurrent Is, determined by first determination module TN1, ismathematically related to rms resonance current value Ir_RMS.

Step 2: Determination of Vr_Max and Vr_Min

Second determination module TN2 determines maximum value Vr_max of thevoltage at the terminals of each resonance capacitor Cr/2 and minimumvalue Vr_min of the voltage at the terminals of each resonance capacitorCr/2 from average value Is_avg of output current Is, determined above,and input voltage (2) Vin. To this end, second determination module TN2uses the expression of a voltage in a resonance capacitor. Sinusoidalvoltage Vr at the terminals of each resonance capacitor Cr/2 is definedby the following formula:V _(r)(t)=½V _(in) +U ₀ cos(2πF _(s) t).

In this equation Vin refers to the measurement of the input voltage ofresonant DC-DC converter 1, U0 is a constant, in particular a constantrepresentative of the amplitude of periodic voltage Vr, Fs refers, asabove, to the switching frequency of transistors T1, T2 and finally trepresents time. There are two identical resonance capacitors Cr/2, afirst capacitor of which connected to an upper terminal of first circuit10-1 and a second capacitor of which connected to a lower terminal offirst circuit 10-1, with both capacitors being connected to their otherterminal in a middle point. Resonant LLC converter circuit is, inparticular, connected firstly to the middle point of resonancecapacitors Cr/2, and secondly to the middle points of transistors T1,T2. According to a small-signal model analysis it is considered thatthese capacitors are mounted in parallel.

Secondly, the expression of the current in a capacitor is known, andtherefore in this case resonance capacitors Cr/2, and the expression ofresonance current Ir:

$\begin{matrix}{{\frac{1}{2}C_{r}\frac{{dV}_{r}(t)}{dt}} = {\frac{1}{2}{I_{r}(t)}}} & (3) \\{et} & \; \\{{I_{r}(t)} = {I_{r\_{peak}}{\sin\left( {2\pi\; F_{s}t} \right)}}} & (4)\end{matrix}$where Ir peak is the maximum value of resonance current Ir. In (3), byreplacing Ir(t) by its expression given in (4), the following isobtained:

$\begin{matrix}{{\frac{1}{2}C_{r}\frac{{dV}_{r}(t)}{dt}} = {\frac{1}{2}I_{r\_{peak}}{\sin\left( {2\;\pi\; F_{s}t} \right)}}} & \left( {3{bis}} \right)\end{matrix}$

In equation (3 bis) voltage Vr is replaced by its expression in (2). Thefollowing is thereby obtained:

$\left. {{\frac{1}{2}C_{r}\frac{{dV}_{r}(t)}{dt}} = {{\frac{1}{2}C_{r}\;\frac{d\left( \left( {{\frac{1}{2}V_{in}} + {U_{0}{\cos\left( {2\;\pi\; F_{s}t} \right)}}} \right) \right)}{dt}} = {{\frac{1}{2}I_{r\_{peak}}{\sin\left( {2\pi\; F_{s}t} \right)}} - {U_{0}2\pi\; F_{s}{Cr}*{\sin\left( {2\pi\; F_{s}t} \right)}}}}} \right) = {I_{r\_{peak}}{\sin\left( {2\;\pi\; F_{s}t} \right)}}$The following is therefore obtained:

$\begin{matrix}{U_{0} = \frac{- I_{r\_{peak}}}{2\;\pi\;{CrF}_{s}}} & (5)\end{matrix}$

Replacing Uo in (1) by its expression in (4) the following is obtained:

$\begin{matrix}{{V_{r}(t)} = {{\frac{1}{2}V_{in}} - {\frac{I_{r\_{peak}}}{2\;\pi\; C_{r}F_{s}}*{\cos\left( {2\;\pi\; F_{s}t} \right)}}}} & \left( {2{bis}} \right)\end{matrix}$

It should be noted that Ir peak=V2 Ir RMS. Thus, in (2 bis), byreplacing Ir peak by its expression, the following is found:

$\begin{matrix}{{V_{r}(t)} = {{\frac{1}{2}V_{in}} - {\frac{\sqrt{2}I_{r\_{RMS}}}{C_{r}2\pi\; F_{s}}{\cos\left( {2\;\pi\; F_{s}t} \right)}}}} & \left( {2{ter}} \right)\end{matrix}$

It is known that Vin and

$\frac{\sqrt{2}I_{r\_{RMS}}}{C_{r}2\pi\; F_{s}}$are constant values. The only variable member of expression (2 ter) ofvoltage Vr is cos(2πF_(s)t). Since the maximum of cos(2πF_(s)t) is equalto 1, and the minimum of a cos(2πF_(s)t) is equal to −1, maximum valueVr_max of voltage Vr and minimum value Vr_min of voltage Vr can bededuced therefrom:

$V_{r\_\max} = {{\frac{1}{2}V_{in}} + \frac{\sqrt{2}I_{r\_{RMS}}}{C_{r}2\pi\; F_{s}}}$$V_{r\_\min} = {{\frac{1}{2}V_{in}} - \frac{\sqrt{2}I_{r\_{RMS}}}{C_{r}2\pi\; F_{s}}}$

From this the expressions of maximum value Vr_max and of minimum valueVr_min are obtained, according to rms resonance current value Ir_RMS.

In addition, the expression of rms resonance current value Ir_RMS as afunction of average value Is_avg of output current Is was previouslydetermined in equation (1 ter). Thus, in the expressions of maximumvalue Vr_max and of minimum value Vr_min rms resonance current valueIr_RMS can be replaced by its expression given in equation (1 ter). Thefollowing is obtained:

$V_{r\_\max} = {{\frac{1}{2}V_{in}} + {\frac{\sqrt{2}}{C_{r}2\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$$V_{r\_\min} = {{\frac{1}{2}V_{in}} - {\frac{\sqrt{2}}{C_{r}2\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$

Step 3: Comparison Between Thresholds and Vr_Min and Vr_Max

Again with reference to FIG. 6, second determination module TN2 alsoincludes a comparison module TNC, which makes the comparison, firstly,between a maximum voltage threshold Vr_define_max and maximum valueVr_max, and secondly between a minimum voltage threshold Vr_define_minand minimum value Vr_min. Said maximum voltage threshold Vr_define_maxis, in particular, defined as the maximum voltage value at the terminalsof resonance capacitor Cr/2, above which the said capacitor isoverloaded. In addition, said minimum voltage threshold Vr_define_minis, in particular, defined as the minimum voltage value at the terminalsof resonance capacitor Cr/2, below which the said resonance capacitor isoverloaded.

Thus, a fault in resonant DC-DC voltage converter 1 is detected whenmaximum voltage value Vr_max is greater than or equal to maximum voltagethreshold Vr_define_max, and/or when minimum voltage value Vr_min isless than or equal to minimum voltage threshold Vr_define_min. In thiscase the fault detection step is the detection of an overload of aresonance capacitor Cr/2.

Again with reference to FIG. 6, when a fault is detected, this isfollowed by a step of disconnection of resonant DC-DC converter 1, whichstops the operation of resonant DC-DC converter 1. To accomplish thisfault detection element UP includes a disconnection element configuredto stop operation of resonant DC-DC converter 1 in the event of a fault.Comparison module TNC thus sends a stop message MS to the disconnectionelement of fault detection element UP of resonant DC-DC converter 1. Thedisconnection element receives said stop message MS, containing a stopcommand, which was previously sent. After receiving this stop message MSthe disconnection element stops the operation of resonant DC-DCconverter 1, thereby protecting resonant DC-DC converter 1 againstdamage to its components, where the said damage would be due to anoverload.

A possible alternative to the step of disconnection of resonant DC-DCconverter 1 consists of a step in which fault detection element UP wouldrequire resonant DC-DC converter 1 to operate in degraded mode but wouldnot order a complete stop of resonant DC-DC converter 1.

The invention claimed is:
 1. An electrical system enabling a directvoltage to be converted into another direct voltage, including: aresonant DC-DC converter including a resonant LLC converter circuitincluding a resonance inductor, two resonance capacitors and atransformer, a control unit including: a first module for determiningthe average value of the output current of the resonant converter over aperiod called the “evaluation period”, a second module for determining amaximum value of the voltage at the terminals of each resonancecapacitor and a minimum value of the voltage at the terminals of eachresonance capacitor over the evaluation period, using the average valueof the output current, a comparison module making a comparison between amaximum voltage threshold and the maximum value of the voltage at theterminals of each resonance capacitor and between a minimum voltagethreshold and the minimum value of the voltage at the terminals of eachresonance capacitor, a fault detection element configured to detect afault in the resonant DC-DC converter if: the said maximum voltage valueis greater than or equal to the maximum voltage threshold, or the saidminimum voltage value is less than or equal to the minimum voltagethreshold.
 2. The electrical system according to claim 1, in which theresonant DC-DC converter includes a rectifier, connected to thesecondary winding of the transformer.
 3. The electrical system accordingto claim 2, in which the first determination module is configured todetermine the average value of the output current, from a measuringpoint located at an output terminal of the rectifier, in particular alow output terminal of the rectifier.
 4. The electrical system accordingto claim 1, in which the second determination module is configured todetermine the maximum voltage value and the minimum voltage value from:the input voltage of the resonant DC-DC converter, the average value ofthe output current, the switching frequency of the resonant DC-DCconverter, the value of the resonance capacitor, the value of outputvoltage Vout of the rectifier, the transformation ratio of thetransformer and the primary magnetising inductor of the transformer. 5.The electrical system according to claim 1, including a filter,connected to the output of the rectifier of the resonant DC-DCconverter.
 6. The electrical system according to claim 1, in which thecomparison module is configured such that the said maximum voltagethreshold is a maximum authorized voltage value at the terminals of theresonance capacitor, above which in particular the said capacitor isoverloaded, and the said minimum voltage threshold is a minimumauthorized voltage value at the terminals of the resonance capacitor,below which in particular the said resonance capacitor is overloaded. 7.The electrical system according to claim 1, in which the fault detectionelement includes a disconnection element configured to stop operation ofthe resonant DC-DC converter in the event of a fault.
 8. A method fordetecting an overload of a resonant DC-DC converter used in anelectrical system including a resonant DC-DC converter including aresonant LLC converter circuit which includes a resonance inductor, tworesonance capacitors and a transformer, where the said method ischaracterised by the fact that it includes the steps of: determination,in particular of measurement, of the output current, determination ofthe average value of the said output current over a period called the“evaluation period”, determination of a maximum value of the voltage atthe terminals of each resonance capacitor, and a minimum value of thevoltage at the terminals of each resonance capacitor over the evaluationperiod “T”, using the average value of the output current, determined inthe previous step, comparison between a maximum voltage threshold andthe maximum value of the voltage at the terminals of each resonancecapacitor and between a minimum voltage threshold and the minimum valueof the voltage at the terminals of each resonance capacitor, detectionof a fault of the resonant DC-DC voltage converter if the said maximumvoltage value is greater than or equal to the maximum voltage threshold,and/or if the said minimum voltage value is less than or equal to theminimum voltage threshold.
 9. The method according to claim 8, in whichthe fault detection step is the detection of an overload of a resonancecapacitor.
 10. The method according to claim 8, including, after thefault detection step, a step of disconnection of the resonant DC-DCconverter, in which the operation of the resonant DC-DC converter isstopped.
 11. The method according to claim 8, in which the maximum valueof the voltage at the terminals of each resonance capacitor isdetermined using the following formula:$V_{r\_\max} = {{\frac{1}{2}V_{in}} + {\frac{\sqrt{2}}{C_{r}2\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$and the minimum value of the voltage at the terminals of each resonancecapacitor is determined using the following formula:$V_{r\_\min} = {{\frac{1}{2}V_{in}} - {\frac{\sqrt{2}}{C_{r}2\pi\; F_{s}}\sqrt{\left( {N \times I_{s\_{avg}} \times \frac{\pi}{2\sqrt{2}}} \right)^{2} + \left( \frac{V_{out}}{4\sqrt{3} \times N \times F_{s} \times L_{m}} \right)^{2}}}}$where Vin is the input voltage of the resonant DC-DC converter, Cr isthe value of the resonance capacitors, Fs is the switching frequency ofthe resonant DC-DC converter, N refers to the transformation ratio ofthe transformer, Is_avrg is the average value of the output current,Vout is the output voltage and Lm refers to the primary magnetizinginductor of the transformer.